(i) Technical Field
The present invention relates to an image processing apparatus, an image processing method, and a non-transitory computer readable medium.
(ii) Related Art
In the related art, in the fields of optical character recognition (OCR) and the like, background elements in a document such as ruled lines are beforehand printed in a specific color, and the specific color is set not to be read by optical processing such as color filtering in such a manner that only black characters that have been written in the document are read. Such processing for eliminating a specific color from an image to be read is called “dropout”, “color dropout”, or the like.
In recent years, dropout of a specific color from color image information, which is obtained by scanning, has been digitally performed. In digital dropout processing, dropout of markings made by using, for example, fluorescent color markers and the like is performed in addition to the above-mentioned dropout of printed ruled lines and backgrounds.
In digital color dropout processing, for example, an image that has been read by a scanner (usually represented in the RGB color space) is converted so as to be represented in the HSL color space (H, S, and L are hue, saturation, and lightness, respectively), and the value of a pixel having a chromatic color (e.g., a pixel having a saturation equal to or higher than a predetermined threshold) is converted into a value that represents the ground color of the image (e.g., white). As a result, characters, figures, and images of black color remain in the image, which is obtained as a result of the conversion, whereas ruled lines and markings each having a high saturation will not remain in the image.
A marking made by using a marker or the like is often superposed with a character, a figure, or an image (hereinafter collectively referred to as “a character or the like”) on a document. In the case where such a marking has been superposed with a character or the like, a portion of the character or the like with which the marking has been superposed may sometimes be eliminated from an image along with the marking through dropout processing. This is because, as a result of the marking having a high saturation being superposed with the portion of the character or the like, the saturation of the portion increases even though the character or the like has black color, and when the dropout processing is performed, the portion that is formed of high saturation pixels is dropped out.